700 sounds like you have 768mb ram.

Likely a 512mb stick and a 256mb stick. The only critical problem with differing speeds, is that the faster one will usually clock down to the lower one, as long as the motherboard supports both. I've heard stories of it shortening ram life, but not critically.
If the motherboard supports it, two 1gb sticks in dual channel, or even two 512mb sticks will do nicely if you are more budget minded. However spending a pretty penny on the graphics card, i'd go for two 1gb sticks in dual channel.

Corsair is a great memory company... but they are a bit pricey. I've got 2x 1gb Kingston DDR400 PC3200 in mine, and it runs well.
For dual channel you'll need exact size and speed matches I believe, and exact matches would be ideal(same brand).

If its an AGP motherboard it could be DDR266, DDR333, DDR400, RDRAM, DDR2 I think... think thats about it.

The motherboard will really help on finding out what memory and card though. I had an older motherboard with a 1800XP processor and the AGP had 4x as the max it could go... So I could not upgrade to a more recent card.... forcing me to get a new motherboard.

A model number on the outside of the case somewhere(sticker or barcode/shrugs... on the back will allow you to search for the computer if its a mainstream and not a custom build).